diff --git a/acsi2stm/Acsi.cpp b/acsi2stm/Acsi.cpp
index c3007dd..1f78b67 100644
--- a/acsi2stm/Acsi.cpp
+++ b/acsi2stm/Acsi.cpp
@@ -38,7 +38,7 @@ void Acsi::onReset() {
 }
 
 void Acsi::refresh() {
-  uint32_t realId = blockDev.mediaId();
+  uint16_t realId = blockDev.mediaId();
   if(realId == mediaId)
     return;
 
diff --git a/acsi2stm/Acsi.h b/acsi2stm/Acsi.h
index e2a0d2f..284128c 100644
--- a/acsi2stm/Acsi.h
+++ b/acsi2stm/Acsi.h
@@ -91,7 +91,7 @@ class Acsi: public Monitor, public Devices {
   SdDev &blockDev;
 
   // Last media ID
-  uint32_t mediaId = 0;
+  uint16_t mediaId = 0;
 
   // SCSI status variables
   ScsiErr lastErr;
diff --git a/acsi2stm/BlockDev.cpp b/acsi2stm/BlockDev.cpp
index 58d65a4..ab069af 100644
--- a/acsi2stm/BlockDev.cpp
+++ b/acsi2stm/BlockDev.cpp
@@ -164,11 +164,11 @@ bool ImageDev::isWritable() {
   return image.isWritable();
 }
 
-uint32_t ImageDev::mediaId(BlockDev::MediaIdMode mode) {
+uint16_t ImageDev::mediaId(BlockDev::MediaIdMode mode) {
   // For now, images cannot be switched on the fly so they cannot change
   // unless the SD card is physically swapped. Derive mediaId from the SD
   // card itself
-  uint32_t id = sd.mediaId(mode);
+  uint16_t id = sd.mediaId(mode);
   if(!id || id != sdMediaId) {
     // No medium or SD card swapped: abort ASAP
     close();
@@ -236,7 +236,7 @@ void SdDev::init() {
     writable = !digitalRead(wpPin);
 #endif
 
-    uint32_t id = mediaId(FORCE);
+    uint16_t id = mediaId(FORCE);
 
     // Open the file system
     image.close();
@@ -437,7 +437,7 @@ bool SdDev::isWritable() {
   return writable;
 }
 
-uint32_t SdDev::mediaId(BlockDev::MediaIdMode mediaIdMode) {
+uint16_t SdDev::mediaId(BlockDev::MediaIdMode mediaIdMode) {
   if(mode == DISABLED)
     return 0;
 
@@ -490,9 +490,9 @@ uint32_t SdDev::mediaId(BlockDev::MediaIdMode mediaIdMode) {
     // init() called us again, just return the updated value
     return lastMediaId;
 
-  uint32_t id = 0;
-  for(unsigned int i = 0; i < sizeof(cid) / 4; ++i)
-    id ^= ((const uint32_t*)&cid)[i];
+  uint16_t id = 0;
+  for(unsigned int i = 0; i < sizeof(cid) / 2; ++i)
+    id ^= ((const uint16_t*)&cid)[i];
 
   // Make sure the same card transfered to another slot won't give the same value
   id += slot * 2;
diff --git a/acsi2stm/BlockDev.h b/acsi2stm/BlockDev.h
index ddd3504..f8e719f 100644
--- a/acsi2stm/BlockDev.h
+++ b/acsi2stm/BlockDev.h
@@ -45,7 +45,7 @@ class BlockDev: public Monitor, public Devices {
   // Return a (hopefully) unique id for this media
   // Returns 0 if no device is present
   // Also serves as a device state detection and refresh
-  virtual uint32_t mediaId(MediaIdMode mode = NORMAL) = 0;
+  virtual uint16_t mediaId(MediaIdMode mode = NORMAL) = 0;
 
   // Size in 512 bytes blocks
   uint32_t blocks;
@@ -76,13 +76,13 @@ class ImageDev: public BlockDev {
   virtual bool writeData(const uint8_t *data, int count = 1);
   virtual bool writeStop();
   virtual bool isWritable();
-  virtual uint32_t mediaId(MediaIdMode mode = NORMAL);
+  virtual uint16_t mediaId(MediaIdMode mode = NORMAL);
 
   SdDev &sd;
   FsBaseFile image;
 
 protected:
-  uint32_t sdMediaId; // SD card owning the current image
+  uint16_t sdMediaId; // SD card owning the current image
 };
 
 // Actual SD card slot
@@ -122,7 +122,7 @@ class SdDev: public BlockDev {
   virtual bool writeData(const uint8_t *data, int count = 1);
   virtual bool writeStop();
   virtual bool isWritable();
-  virtual uint32_t mediaId(MediaIdMode = NORMAL);
+  virtual uint16_t mediaId(MediaIdMode = NORMAL);
 
   // Permanently disable the slot
   void disable();
@@ -149,7 +149,7 @@ class SdDev: public BlockDev {
   friend class ImageDev;
 protected:
   static const uint32_t mediaCheckPeriod = 500;
-  uint32_t lastMediaId;
+  uint16_t lastMediaId;
   uint32_t lastMediaCheckTime;
   void reset();
 };
diff --git a/acsi2stm/GemDrive.cpp b/acsi2stm/GemDrive.cpp
index 261e233..80df243 100644
--- a/acsi2stm/GemDrive.cpp
+++ b/acsi2stm/GemDrive.cpp
@@ -2127,7 +2127,7 @@ GemDrive * GemDrive::getDrive(uint8_t id) {
   return nullptr;
 }
 
-GemDrive * GemDrive::getDrive(uint32_t mediaId, BlockDev::MediaIdMode mode) {
+GemDrive * GemDrive::getDrive(uint16_t mediaId, BlockDev::MediaIdMode mode) {
   if(!mediaId)
     return nullptr;
 
diff --git a/acsi2stm/GemDrive.h b/acsi2stm/GemDrive.h
index ec12a8a..4ae48f7 100644
--- a/acsi2stm/GemDrive.h
+++ b/acsi2stm/GemDrive.h
@@ -93,13 +93,14 @@ struct TOS_PACKED GemPattern {
 // GemDrive DTA, compatible with TOS DTA
 struct TOS_PACKED GemDriveDTA {
   TinyFile file;
+  uint8_t attribMask;
+  uint8_t unused;
   GemPattern pattern;
   uint8_t d_attrib;
   Word d_time;
   Word d_date;
   Long d_length;
-  char d_fname[13];
-  uint8_t attribMask;
+  char d_fname[14];
 };
 
 struct GemPath: public FsFile {
@@ -141,7 +142,7 @@ struct GemPath: public FsFile {
   uint16_t indexes[maxDepth];
   SdDev &sd;
 public:
-  uint32_t mediaId;
+  uint16_t mediaId;
 };
 
 struct GemFile: public TinyFile {
@@ -211,7 +212,7 @@ struct GemDrive: public Devices, public Tos {
   static GemDrive * getDrive(char *path, char **outPath = nullptr);
   static GemDrive * getDrive(Long pathAddr, char **outPath = nullptr);
   static GemDrive * getDrive(uint8_t driveId);
-  static GemDrive * getDrive(uint32_t mediaId, BlockDev::MediaIdMode mode = BlockDev::NORMAL);
+  static GemDrive * getDrive(uint16_t mediaId, BlockDev::MediaIdMode mode = BlockDev::NORMAL);
   static void closeProcessFiles();
   static oflag_t attribToSdFat(uint8_t attrib);
   static bool ownFd(Word fd);
diff --git a/acsi2stm/TinyFile.cpp b/acsi2stm/TinyFile.cpp
index d6c1a9b..bd7411a 100644
--- a/acsi2stm/TinyFile.cpp
+++ b/acsi2stm/TinyFile.cpp
@@ -24,15 +24,16 @@
 #include "TinyFile.h"
 
 TinyFile::TinyFile() : index(0) {
+
 }
 
-void TinyFile::set(uint32_t mediaId_, FsFile &parent, FsFile &file) {
+void TinyFile::set(uint16_t mediaId_, FsFile &parent, FsFile &file) {
   mediaId = mediaId_;
   dirCluster = getCluster(parent);
   index = file.dirIndex() + 1;
 };
 
-void TinyFile::set(uint32_t mediaId_, FsFile &parent) {
+void TinyFile::set(uint16_t mediaId_, FsFile &parent) {
   mediaId = mediaId_;
   dirCluster = getCluster(parent);
   index = 0;
@@ -150,7 +151,7 @@ void TinyFile::closeLast() {
   lastMediaId = 0;
 }
 
-void TinyFile::ejected(uint32_t mediaId) {
+void TinyFile::ejected(uint16_t mediaId) {
   // Called on SD card hot swap
   if(mediaId == lastMediaId) {
     lastFile = FsFile();
@@ -161,4 +162,4 @@ void TinyFile::ejected(uint32_t mediaId) {
 
 FsFile TinyFile::lastFile;
 FsFile TinyFile::lastParent;
-uint32_t TinyFile::lastMediaId;
+uint16_t TinyFile::lastMediaId;
diff --git a/acsi2stm/TinyFile.h b/acsi2stm/TinyFile.h
index 50b0fce..985e51e 100644
--- a/acsi2stm/TinyFile.h
+++ b/acsi2stm/TinyFile.h
@@ -38,10 +38,10 @@ struct __attribute__((__packed__)) TinyFile {
   }
 
   // Point this TinyFile at a file
-  void set(uint32_t mediaId, FsFile &parent, FsFile &file);
+  void set(uint16_t mediaId, FsFile &parent, FsFile &file);
 
   // Point this TinyFile at the beginning of a folder
-  void set(uint32_t mediaId, FsFile &parent);
+  void set(uint16_t mediaId, FsFile &parent);
 
   // Acquires the file.
   // If mediaId is set, enables the acquire cache, speeding up things a lot.
@@ -62,7 +62,7 @@ struct __attribute__((__packed__)) TinyFile {
   static uint32_t getCluster(FsFile &file);
   static void setCluster(FsFile &file, uint32_t cluster);
 
-  uint32_t mediaId;
+  uint16_t mediaId;
   uint32_t dirCluster;
   uint16_t index;
 
@@ -72,11 +72,11 @@ struct __attribute__((__packed__)) TinyFile {
   static const uint16_t VOLUME = 0xfffd;
 
   static void closeLast();
-  static void ejected(uint32_t mediaId);
+  static void ejected(uint16_t mediaId);
 
   static FsFile lastFile;
   static FsFile lastParent;
-  static uint32_t lastMediaId;
+  static uint16_t lastMediaId;
 };
 
 #endif